Cross-point diode arrays and methods of manufacturing cross-point diode arrays

ABSTRACT

Methods of forming an array of memory cells and memory cells that have pillars. Individual pillars can have a semiconductor post formed of a bulk semiconductor material and a sacrificial cap on the semiconductor post. Source regions can be between columns of the pillars, and gate lines extend along a column of pillars and are spaced apart from corresponding source regions. Each gate line surrounds a portion of the semiconductor posts along a column of pillars. The sacrificial cap structure can be selectively removed to thereby form self-aligned openings that expose a top portion of corresponding semiconductor posts. Individual drain contacts formed in the self-aligned openings are electrically connected to corresponding semiconductor posts.

TECHNICAL FIELD

The present technology is related to cross-point diode arrays and othertypes of structures used in memory devices or other types ofmicroelectronic devices, and methods for manufacturing such structures.

BACKGROUND

The microelectronic industry is under intense pressure to produce highperformance devices in small package sizes at continually decreasingcosts. Smart phones, portable computers, digital cameras, portable musicand media players, and many other electronic products require fastermemory devices with more capacity. As such, memory device manufacturersin particular seek reliable, low-cost processes for fabricatinghigh-performance devices.

Memory devices may have large arrays of memory cells, and reducing thesize of individual memory cells provides a concomitant increase in thebit density of the memory devices. Cross-point memory cells are locatedin the vertical overlap regions between word lines and bit lines.Cross-point memory cells include structures that undergo a stable anddetectable change when exposed to a current between a word line and acorresponding bit line. Because cross-point memory cells are located inthe overlap regions between bit lines and word lines, these memory cellsmay theoretically be formed to extremely small dimensions.

One concern of manufacturing cross-point memory cells is that thesmallest features of the structures can be difficult to form usingphotolithography processes. For example, it is difficult to pattern thewafers to form the drain structures of a cross-point memory cell becausethese features are so small that they cannot be formed reliably usingexisting photolithography processes. It is also difficult tophoto-pattern gate structures that completely surround the individualpillars in high density cross-point memory arrays. Moreover, it is veryexpensive to form very small features using photolithography processesbecause these processes require expensive equipment and materials.Additional concerns of cross-point memory arrays include reversedleakage current, serial resistance, and accurate control ofcurrent-voltage uniformity that are important for multi-level cellconfigurations. Therefore, it is desired to develop improved methods forforming highly integrated circuitry, and in particular cross-pointmemory cells, and to develop improved circuitry for such features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is flow chart of an embodiment of a method for forming an arrayof memory cells in accordance with the technology.

FIG. 2A is a cross-sectional view and FIG. 2B is a top-plan view of anembodiment of a substrate at a stage of a method for forming an array ofmemory cells in accordance with the technology.

FIG. 3A is a cross-sectional view and FIG. 3B is a top-plan view of anembodiment of a substrate at another stage of a method for forming anarray of memory cells in accordance with the technology.

FIG. 4A is a cross-sectional view and FIG. 4B is a top-plan view of anembodiment of a substrate at another stage of a method for forming anarray of memory cells in accordance with the technology.

FIG. 5A is a cross-sectional view and FIG. 5B is a top-plan view of anembodiment of a substrate at another stage of a method for forming anarray of memory cells in accordance with the technology.

FIG. 6A is a cross-sectional view and FIG. 6B is a top-plan view of anembodiment of a substrate at another stage of a method for forming anarray of memory cells in accordance with the technology.

FIG. 7A is a cross-sectional view and FIG. 7B is a top-plan view of anembodiment of a substrate at another stage of a method for forming anarray of memory cells in accordance with the technology.

FIG. 8A is a cross-sectional view and FIG. 8B is a top-plan view of anembodiment of a substrate at another stage of a method for forming anarray of memory cells in accordance with the technology.

FIGS. 9A and 9B are a cross-sectional views and FIG. 9C is a top-planview of an embodiment of a substrate at another stage of a method forforming an array of memory cells in accordance with the technology.

FIG. 10 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 11 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 12 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 13 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 14 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 15 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 16 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 17 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 18 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 19 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 20 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 21 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 22 is a cross-sectional view of an embodiment of a substrate atanother stage of a method for forming an array of memory cells inaccordance with the technology.

FIG. 23 is a flow chart of another embodiment of a method for formingvertical memory cells in accordance with the technology.

DETAILED DESCRIPTION

Specific details of several embodiments of the new technology aredescribed below with reference to memory cell arrays and methods formanufacturing arrays of memory cells. Many embodiments, for example, aredirected to cross-point memory arrays used in phase change memory orother types of memory. Memory cells and other semiconductor componentsare manufactured on and/or in semiconductor wafers that can includesemiconductor substrates upon which and/or in which microelectronicdevices, micromechanical devices, data storage elements, optics,read/write components, and other features can be fabricated. Forexample, SRAM, DRAM (e.g., DDR/SDRAM), flash memory (e.g., NANDflash-memory), phase change memory (PCRAM), processors, imagers, lightemitting diodes (LEDs) and other types of devices can be constructed onsemiconductor wafers. Although many of the embodiments are describedbelow with respect to semiconductor devices that have integratedcircuits, other types of devices manufactured on other types ofsubstrates may be within the scope of the new technology. Moreover,several other embodiments of the new technology can have differentconfigurations, components, or procedures than those described in thissection. A person of ordinary skill in the art, therefore, willaccordingly understand that the new technology may have otherembodiments with additional elements, or the new technology may haveother embodiments without several of the features shown and describedbelow with reference to FIGS. 1-24.

FIG. 1 is a flow chart of an embodiment of a method 100 of forming anarray of memory cells in accordance with the technology. The method caninclude forming a plurality of pillars in an array of rows and columns(block 110). The pillars are formed from a substrate of a bulksemiconductor material, and individual pillars have a sacrificial capstructure. For example, individual pillars can have a semiconductor postformed of the bulk semiconductor material and the sacrificial capstructure can be on each of the semiconductor posts. The method furtherincludes forming source regions between columns of the pillars (block120), and forming conductive gate lines (block 130). Individual gatelines extend along a column of pillars and are spaced apart fromcorresponding source regions. Each gate line completely surrounds aportion of the semiconductor posts along a corresponding column ofpillars. This embodiment of the method 100 further includes selectivelyremoving the sacrificial cap structure and thereby forming self-alignedopenings that expose a top portion of corresponding semiconductor posts(block 140), and forming individual drain contacts in the self-alignedopenings that are electrically connected to corresponding semiconductorposts (block 150).

FIGS. 2A and 2B are cross-sectional and top plan views, respectively, ofan embodiment of a substrate 200 (e.g., a wafer) at a stage of themethod 100. At this stage, the substrate 200 has a bulk semiconductormaterial 210 (FIG. 2A), a plurality of shallow trench isolation (STI)structures 220, and a sacrificial structure 230 (FIG. 2A). The STIstructures 220 can have array trenches 222, peripheral trenches 224, adielectric liner 226 lining the array and peripheral trenches 222 and224, and an oxide 228 filling the array and peripheral trenches. Thearray trenches 222 define the spacing between rows of pillars that formindividual memory cells or memory units.

The sacrificial structure 230 can include a first sacrificial material232 and an optional second sacrificial material 234. The firstsacrificial material 232 can be selectively removable relative to thesecond sacrificial material 234 using a suitable solvent or othermaterial, and the first and second sacrificial material 232 and 234 canalso provide separate stop-on features for mechanical removal processes.For example, the first sacrificial material 232 can be a polymericmaterial and the second sacrificial material 234 can be a nitride. Thesacrificial structure 230 can be separated from the bulk semiconductormaterial 210 by a dielectric layer 236. In one specific embodiment, thedielectric layer 236 can be a silicon oxide layer having a thickness ofapproximately 50 angstroms, the first sacrificial material 232 can be apolymeric material having a thickness of about 800 angstroms, and thesecond sacrificial material 234 can be a nitride having a thickness ofapproximately 400 angstroms.

FIGS. 3A and 3B are cross-sectional and top plan views, respectively, ofan embodiment of the substrate 200 at a subsequent stage of the method100 in which column trenches 240 are formed. The column trenches 240 canbe perpendicular or at an oblique angle relative to the oxide 228 in thearray trenches 222. The column trenches 240 can be formed byphoto-patterning a resist or other material (not shown) on top of thesacrificial structure 230 using photolithography or other techniquesknown in the art. The array trenches 222 can have a first width W₁ (FIG.3B), and the column trenches 240 have a second width W₂ greater than thefirst width W₁ of the array trenches 222. The column trenches 240 areformed by etching through the sacrificial structure 230 and into thesemiconductor material 210 to a desired trench depth. The etchingprocess can be a dry etch or other method known in the art. In oneembodiment, the target trench depth of the column trenches 240 isapproximately equal to the depth of the array trenches 222 andperipheral trenches 224 of the STI structure 220.

The substrate 200 has an array of pillars 250 that provide verticalchannels for forming individual memory cells. The oxide 228 in the arraytrenches 222 supports the pillars 250. Referring to FIG. 3A, individualpillars 250 can include a semiconductor post 252 of the bulksemiconductor material 210 and a sacrificial cap 254 formed from theremaining portions of the first and second sacrificial material 232 and234. The individual semiconductor posts 252 can each include a proximalportion 256 a, an intermediate section 256 b, and a distal portion 256 crelative to the base of the bulk semiconductor material 210 at thebottom of the column trenches 240.

The array of pillars 250 can be arranged in a plurality of rows (R₁, R₂,etc.) and a plurality of columns (C₁, C₂, C₃, C₄, etc.). The rows ofpillars 250 are spaced apart from each other by the first width W₁ ofthe array trenches 222, and the columns of pillars 250 are spaced apartfrom each other by the second width W₂ of the column trenches 240. Asdescribed in more detail below, the first width W₁ and second width W₂are selected based upon the thickness of a conductive gate material suchthat the resulting conductive gate lines completely surround a portionof each of the pillars 250. The first width W₁, for example, can be lessthan 200% of the thickness of the conductive gate material, and thesecond width W₂ of the column trenches 240 can be greater than 200% ofthe thickness of the conductive gate material. In more specificembodiments, the second width W₂ can be about 300% greater than thethickness of the conductive gate material.

FIGS. 4A-6B illustrate embodiments of the substrate 200 at subsequentstages of the method 100. FIGS. 4A and 4B are cross-sectional and topplan views, respectively, after forming a first dielectric liner 257 anda second dielectric liner 258 along the sidewalls of the pillars 250.The first dielectric liner 257 can be formed by growing an oxide viaoxidation, and the second dielectric liner 258 can be formed by growingor otherwise depositing a thin blocking spacer material. For example,the first dielectric liner 257 can be an oxide having a thickness ofapproximately 20 angstroms, and the second dielectric liner 258 can be adeposited TEOS having a thickness of approximately 50-100 angstroms. Thefirst and second dielectric liners 257 and 258 are then spacer etched toremove the portions of the liners on the top of the pillars 250 and inthe bottom of the column trenches 240. The method can further include anactivation procedure to re-crystallize the semiconductor material.

After spacer etching the first and second liners 257 and 258, sourceregions 260 are formed by implanting the desired implant species intothe bulk semiconductor material 210 at the bottom of the column trenches240. The source regions 260, for example, can be an N-type species.FIGS. 5A and 5B are cross-sectional and top plan views, respectively, ofthe substrate 200 at a subsequent stage in which a silicide 262 isformed at the source regions 260 by exposing the implanted material to asource, such as Ni, Co, or Ti, that selectively forms a metal silicide.During the silicide process, the sidewalls of the pillars 250 areprotected by the second dielectric liner 258 and the second sacrificialmaterial 234 of the cap structure 254. The un-reacted metal can beremoved from the silicide 262 with a wet etch or other technique, and athin nitride liner can be formed to cap the silicide 262

FIGS. 6A and 6B are cross-sectional and top plan views, respectively, ofan embodiment of the substrate 200 at a later stage in which the columntrenches 240 are filled with an oxide 248 and then planarized using achemical-mechanical planarization process or other process that stops onthe second sacrificial material 234 of the sacrificial cap 254. Theoxide 248 that fills the column trenches 240 can be the same as theoxide 228 that fills the array trenches 222 of the STI structure 220.The thin nitride cap deposited on the silicide 262 protects the silicid262 from subsequent gate oxidation.

FIGS. 7A-8B illustrate embodiments of additional stages of the method100. FIGS. 7A and 7B are cross-sectional and top plan views,respectively, of the substrate 200 after removing the second sacrificialmaterial 234 and a portion of the oxides 228 and 248. The secondsacrificial material 234 can be selectively removed relative to theoxides 228 and 248 in a separate process, or the second sacrificialmaterial 234 and the oxides 228 and 248 can be removed in the sameprocess. The oxides 228 and 248 are accordingly recessed relative to thedistal portion 256 c of the semiconductor posts 252. In one embodiment,the thickness of the remaining portions of the oxides 228 and 248 isabout 700 angstroms.

FIGS. 8A and 8B are cross-sectional and top plan views, respectively,illustrating the substrate 200 at a further stage in which more materialis removed from the oxides 228 and 248 and the sidewalls of the pillars250 are cleaned. The remaining portions of the oxides 228 and 248provide a dielectric spacer between the source regions 260 and theintermediate sections 256 b of the semiconductor posts 252 where themetal gates will be formed. In one embodiment, the thickness of theremaining portions of the oxide 228 and 248 is about 200 angstroms. Theexposed sidewalls of the semiconductor posts 252 provide a cleansemiconductor surface for forming a gate dielectric 265. The gatedielectric 265 can be formed of conventional SiO₂, nitride hardenedSiO₂, or other suitable materials. The portions of the gate dielectric265 at the intermediate sections 256 b of the posts 252 define the gatedielectric for the memory cells.

FIGS. 9A-9C illustrate embodiments of the substrate 200 at ametallization stage of the method 100. FIGS. 9A and 9B arecross-sectional views taken along lines 9A-9A and 9B-9B of FIG. 9C. Themethod further includes depositing a conductive gate material 270 overthe pillars 250 and into the recessed portions of the array trenches 222(FIG. 9B) and the column trenches 240 (FIG. 9A). The gate material 270has a thickness T (FIG. 9A) such that the smaller first width W₁ of thearray trenches 222 causes the gate material 270 to pinch off atintersections 271 (FIG. 9C) over the portions of the oxide 228 betweenrows of pillars 250. The thickness T of the gate material 270, however,is not so thick that it spans the larger second width W₂ of the columntrenches 240. The gate material 270 can be tantalum nitride or othersuitable materials.

FIG. 10 is a cross-sectional view illustrating embodiments of thesubstrate 200 at a stage of the method 100 in which gate lines 272 areformed from the gate material 270. The gate lines 272 are formed byspacer etching the gate material 270 (FIGS. 9A-9C) until the remainingportions of the gate material 270 are about at or near the intermediatesections 256 b (FIG. 5A) of the semiconductor posts 252. The spacer etchcan selectively stop on the oxide 248 at the bottom of the columnchannels 240. However, because the gate material 270 completely filledthe array tranches 222 between pillars 250, a portion of the gatematerial 270 remains in the bottom of the array trenches 222 between thepillars 250. Each metal gate line 272 accordingly completely surroundsthe corresponding intermediate sections 256 b of the posts 252 along acolumn of pillars 250. After forming the gate lines 272, an optionalstep includes implanting an n-type implant or other species into thedistal portions 256 c of the semiconductor posts using an angle implant(e.g., an n-type lightly doped drain).

FIG. 11 is a cross-sectional view of embodiments of the substrate 200 atanother stage of the method 100. At this stage dielectric spacers 274are formed above the gate lines 272, along the sidewalls of the pillars250. The dielectric spacers 274 can be formed by depositing a dielectricmaterial over the substrate and spacer etching the dielectric materialto remove it from horizontal or other non-vertical surfaces. Thedielectric spacers 274, for example, can be a nitride that isselectively etchable relative to the gate dielectric 265 and the oxide248 in the bottom of the column channels 240.

FIG. 12 is a cross-sectional view of embodiments of the substrate 200 ata subsequent stage that includes depositing an oxide 278 into the openportions of the array trenches 222 and the column trenches 240. Theoxide 278 can be planarized to remove (a) an over burden of the oxide278 and (b) the portions of the gate dielectric 265 on top of theremaining portions of the first sacrificial material 232. Theplanarization procedure accordingly exposes the remaining portions ofthe first sacrificial dielectric layer 232 without any additionalphotolithography procedures. As such, the exposed portions of the firstsacrificial material 232 over the individual semiconductor posts 252 are“self-aligned” in that a separate photolithography process is notnecessary to form a pattern corresponding to the exposed portions of thefirst sacrificial material 232.

FIG. 13 is a cross-sectional view illustrating embodiments of subsequentstages in which the exposed portions of the first sacrificial material232 are selectively etched relative to the oxide 278 to formself-aligned openings 280 directly over the distal portions 256 c ofcorresponding semiconductor posts 252. The first sacrificial material232 is formed from a material that is selectively removable relative tothe oxide 278 and the dielectric spacer 274 such that the self-alignedopenings 280 can be formed directly over the distal portions 256 c ofthe semiconductor post 252 without having to align openings of a maskwith the posts 252 (e.g., a mask photo-patterned on the wafer to haveopenings aligned with the posts). An n-type or other species can beimplanted into the distal portions 256 c of the semiconductor posts 252to form drain regions 282. The drain regions 282 can accordingly beformed by directly implanting the implant species through the openings280 at this stage of the process either in lieu of or in addition toforming the drain regions 282 earlier in the method using an angledimplant process as described above with respect to in FIG. 10.

FIG. 14 is a cross-sectional view illustrating embodiments of anadditional stage of the method 100 in which a plurality of drain contactplugs 284 are formed in the openings 280 (FIG. 13). The drain contactplugs 284 can be formed by depositing a metal or other suitablyconductive material into the openings 280 (FIG. 13), and then removingan overburden portion of the metal from the top of the oxide 278 using achemical-mechanical planarization or other process. The drain contactplugs 284, for example, can be tungsten or other suitable metals. Thedrain contact plugs 284 are accordingly mask-less, self-aligned featuresthat are aligned with the semiconductor posts 252. At this point, thestructure of each pillar 250 defines a memory cell, and the array ofmemory cells is ready for cell integration into a PCRAM or other type ofmemory device.

Several embodiments of the method 100 and the resulting substrate 200can be used to form very small cross-point memory cells. Some of thesmallest features of a cross-point cell array are the gate lines and thecontact plugs, and current photolithographic processes may not be ableto efficiently form these features at the smallest sizes. The specificembodiment of the method 100 shown and described above, however, enablesthe gate lines and contact plugs to be formed without having to form apattern of either the gate lines or the contact plugs usingphotolithography. This not only eliminates the process errors that canbe induced by photolithographically patterning very small features, butis also reduces the utilization of expensive photolithography equipment.Therefore, several embodiments of the method 100 and substrate 200provide a cost effective process for fabricating cross-point memorycells.

Several embodiments of the method 100 and the substrate 200 also providea structure that can address other concerns of cross-point memory cells.For example, because the gate lines 272 surround the intermediatesections 256 b of the semiconductor posts 252 along a column of pillars250, the issues of reversed leakage current, serial resistance along acolumn of pillars 250, and current-voltage control are expected to bemitigated.

FIGS. 15-22 illustrate stages of another embodiment of the method 100.FIG. 15 is a cross-sectional view that illustrates a stage immediatelyafter the stages described above with respect to FIGS. 2A-9C. FIG. 15,more specifically, illustrates a stage of another embodiment of theprocess 100 in which the gate material 270 has been spacer etched toselectively stop on the gate dielectric 265 and the oxide 248. Theprocess illustrated in FIG. 15 differs from that illustrated in FIG. 10in that the gate material 270 shown in FIG. 15 is not etched to the sameextent as it is in FIG. 10. The stage illustrated in FIG. 15 accordinglyforms an intermediate structure 302 for forming gate lines that surroundthe individual pillars 250. In several embodiments, the gate lines 272completely surround the individual pillars 250.

FIG. 16 is a cross-sectional view illustrating embodiments at a laterstage in which an oxide 308 is deposited into the open portions of thecolumn trenches 240. At this stage an overburdened portion of the oxide308 is removed using a chemical-mechanical planarizing procedure orother process that stops on the first sacrificial material 232. Theremoval procedure accordingly removes portions of the gate dielectric265 from the top surface of the remaining portions of the firstsacrificial material 232 in alignment with the individual pillars 250.

FIG. 17 is a cross-sectional view illustrating subsequent stages inwhich the remaining portions of the first sacrificial material 232 areremoved from the regions over the pillars 250 to form self-alignedopenings 310 aligned with corresponding pillars 250. The firstsacrificial material 232 can be selectively removed such that theremaining portions of the gate dielectric 265 remain intact. FIG. 18 isa cross-sectional view that shows a subsequent stage in which theremaining portions of the gate dielectric 265 are removed to expose thedistal portions 256 c of the semiconductor posts 252. At this point adesired implant species can be implanted into the distal portions 256 ofthe semiconductor posts 252 to form drain regions 282.

FIG. 19 is a cross-sectional view illustrating a subsequent stage thatincludes removing a portion of the intermediate structure 302 to formthe individual gate lines 272 at the intermediate sections 256 b ofcorresponding semiconductor posts 252. The gate lines 272 are formedsuch that they are spaced apart from both the drain regions 282 and thesource regions 260. The gate lines 272 also surround the intermediatesections 256 b of the semiconductor posts 252 along a correspondingcolumn of the pillars 250 as described above with respect to FIG. 10.

FIGS. 20-22 are cross-sectional views illustrating additional stages ofthis embodiment of the method. FIG. 20 illustrates a stage in which adielectric spacer 311 is deposited or otherwise formed in the substrate200. The dielectric spacer 311 can be a nitride, and the thickness ofthe dielectric spacer 311 can be selected to fill the gaps where thegate metal was recessed to form the gate lines 272. FIG. 21 illustratesspacer etching the dielectric spacer 311 to expose the drain regions282. More specifically, the dielectric spacer 311 can be spacer etchedto form dielectric spacers 314 above the gate lines 272 and along thesidewalls of the oxide 308. The drain regions 282 are accordinglyexposed. FIG. 22 illustrates the substrate 200 at a subsequent stage inwhich contact plugs 284 are formed in the openings over the drainregions 282.

FIG. 23 is a flow chart of a method 400 for forming an array of verticalmemory cells in accordance with another embodiment of the technology. Inone embodiment, the method 400 includes forming a plurality of pillars(block 410) in which each pillar has a semiconductor post and asacrificial cap. The pillars are arranged in rows spaced apart by a rowspacing and columns spaced apart by a column spacing larger than the rowspacing. The method 400 can further include forming source regions at aproximal region of the semiconductor posts (block 420) and formingconductive gate lines that completely surround an intermediate sectionof the semiconductor posts in a corresponding column of pillars (block430). After forming the gate lines, the sacrificial caps can be removedselectively without forming a photo-pattern corresponding to a patternof the sacrificial caps and thereby forming self-aligned openings over adistal portion of each of the semiconductor posts (block 440). Themethod 400 can also include forming drain contacts in the self-alignedopenings that are electrically connected to corresponding semiconductorposts (block 450).

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but well-known structures and functions have not been shown or describedin detail to avoid unnecessarily obscuring the description of theembodiments of the invention. Where the context permits, singular orplural terms may also include the plural or singular term, respectively.Moreover, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twoor more items, then the use of “or” in such a list is to be interpretedas including (a) any single item in the list, (b) all of the items inthe list, or (c) any combination of the items in the list. Additionally,the term “comprising” is used throughout to mean including at least therecited feature(s) such that any greater number of the same featureand/or additional types of features are not precluded Accordingly, theinvention is not limited except as by the appended claims.

We claim:
 1. A method of forming an array of memory cells, comprising:forming a plurality of pillars comprising fabricating a shallow trenchisolation structure having array trenches and an oxide in the arraytrenches, forming a column trench mask, and etching column trenches inthe semiconductor material that space apart the columns of the pillars,wherein individual pillars have a semiconductor post formed of asemiconductor material and a sacrificial cap on the semiconductor post;forming source regions between columns of the pillars; forming aplurality of gate lines, wherein individual gate lines extend along acorresponding column of pillars and are spaced apart from correspondingsource regions, and wherein each gate line surrounds a portion of thesemiconductor posts along a column of pillars; selectively removing thesacrificial caps and thereby forming self-aligned openings that expose atop portion of corresponding semiconductor posts; and forming individualdrain contacts in the self-aligned openings that are electricallyconnected to corresponding semiconductor posts.
 2. The method of claim 1wherein forming the pillars comprises fabricating a shallow trenchisolation structure having array trenches and an oxide in the arraytrenches and etching column trenches in the semiconductor materialtransverse to the array trenches.
 3. The method of claim 1 wherein thearray trenches have a first width and the column trenches have a secondwidth greater than the first width.
 4. The method of claim 1 wherein thesacrificial cap structure comprises a first sacrificial material and asecond sacrificial material, wherein the first and second sacrificialmaterials are selectively removable relative to each other.
 5. Themethod of claim 1 wherein the pillars are spaced apart from each otherby array trenches between the pillars and column trenches.
 6. The methodof claim 5, wherein forming the gate lines comprises depositing a metalinto the array trenches and the column trenches such that the metalcompletely surrounds at least an intermediate section of each of theposts, and wherein the metal deposited into the column trenches does notspan the entire width of the column trenches.
 7. The method of claim 6,further comprising spacer etching the metal, forming a dielectric spacermaterial over the metal gate structures and the pillars, spacer etchingthe dielectric spacer material, filling the column trenches with anoxide, removing a portion of the oxide to expose portions of thesacrificial caps, and wherein selectively removing the sacrificial capscomprises selectively etching the sacrificial caps relative to theoxide.
 8. The method of claim 7 wherein the oxide is removed bychemical-mechanical planarization.
 9. The method of claim 7, furthercomprising implanting a drain into a top portion of each semiconductorpost before forming the drain contacts.
 10. The method of claim 1,further comprising directly implanting a drain into a distal portion ofeach semiconductor posts through the self-aligned openings.
 11. A methodof forming an array of memory cells, comprising: forming a plurality ofpillars comprising fabricating a shallow trench isolation structurehaving array trenches and an oxide in the array trenches, forming acolumn trench mask, and etching column trenches in the semiconductormaterial that space apart the columns of the pillars, wherein individualpillars have a semiconductor post formed of a semiconductor material anda sacrificial cap on the semiconductor post; forming source regionsbetween columns of the pillars, wherein forming the source regionscomprises implanting an N-type implant into the semiconductor materialat a bottom of the column trenches; forming a plurality of gate lines,wherein individual gate lines extend along a corresponding column ofpillars and are spaced apart from corresponding source regions, whereineach gate line surrounds a portion of the semiconductor posts along acolumn of pillars, and wherein forming the gate lines comprises fillingthe column trenches with an oxide, etching the oxide in the arraytrenches and the column trenches to a depth spaced apart from the sourceregions, depositing a metal into the array trenches and the columntrenches such that the metal fills the array trenches but does not fillthe column trenches, and spacer etching the metal to a level spacedapart from the sacrificial caps; selectively removing the sacrificialcaps and thereby forming self-aligned openings that expose a top portionof corresponding semiconductor posts, wherein selectively removing thesacrificial caps comprises etching away the sacrificial caps withoutphoto-patterning a mask with a pattern of openings corresponding to thepattern of the self-aligned openings; and forming individual draincontacts in the self-aligned openings that are electrically connected tocorresponding semiconductor posts.
 12. A method of forming an array ofvertical memory cells, comprising: forming a plurality of pillars,wherein each pillar has a semiconductor post and a sacrificial cap, andwherein the pillars are arranged in rows spaced apart by a row spacingand columns spaced apart by a column spacing larger than the rowspacing; forming source regions at a proximal region of thesemiconductor posts; forming gate lines that surround an intermediatesection of the semiconductor posts; selectively removing the sacrificialcaps comprising etching away the sacrificial caps without forming a maskcorresponding to a pattern of the sacrificial caps, thereby formingself-aligned openings over a distal portion of each of the semiconductorposts; and forming drain contacts in the self-aligned openings that areelectrically connected to corresponding semiconductor posts.
 13. Themethod of claim 12 wherein forming the pillars comprises fabricating ashallow trench isolation structure having array trenches and an oxide inthe array trenches and etching column trenches in the semiconductormaterial transverse to the array trenches.
 14. The method of claim 12wherein forming the gate lines comprises depositing a metal having athickness into the array trenches and the column trenches such that themetal fills the array trenches but does not fill the column trenches,and spacer etching the metal to a level spaced apart from thesacrificial caps.
 15. The method of claim 14 wherein the array trencheshave a first width less than twice the thickness of the metal and thecolumn trenches have a second width greater than the first width. 16.The method of claim 12 wherein selectively removing the sacrificial capcomprises planarizing a first sacrificial material and then etching asecond sacrificial material.
 17. The method of claim 12 wherein thesacrificial caps comprise a first sacrificial material and a secondsacrificial material, wherein the first and second sacrificial materialsare selectively removable relative to each other.
 18. The method ofclaim 14, further comprising spacer etching the metal, forming adielectric spacer material over the metal gate structures and thepillars, spacer etching the dielectric spacer material, filling thecolumn trenches with an oxide, removing a portion of the oxide to exposeportions of the sacrificial cap structure, and wherein selectivelyremoving the sacrificial caps comprises selectively etching thesacrificial caps relative to the oxide.
 19. The method of claim 12,further comprising implanting a drain into a top portion of eachsemiconductor post before forming the drain contacts.
 20. The method ofclaim 19, wherein the drains are angle implanted into the distalportions of the semiconductor posts before selectively removing thesacrificial caps.
 21. The method of claim 19, wherein the drains areimplanted through the self-aligned openings and into the semiconductorposts.